The term 'cortical blindness' indicates loss of sight due to bilateral lesions of the occipital lobes. It is a rare, but severe, side effect of chemotherapeutic agents. Cortical blindness was diagnosed in a 67 year old woman with leiomyosarcoma of the large bowel, treated by vincristine-containing chemotherapy. Cortical blindness without focal neurological signs and with two repeated normal brain computed tomography scans, in which there was no structural damage to the occipital lobes, suggests a metabolic or toxic reaction as a cause in our patients. The temporal relationship between vincristine treatment and cortical blindness implicates vincristine as the possible causative agent for this catastrophic phenomenon.